Enhanced Lithium Storage Performance of α-MoO3/CNTs Composite Cathode

Orthorhombic molybdenum oxide (α-MoO3), as a one-layered pseudocapacitive material, has attracted widespread attention due to its high theoretical lithium storage specific capacity (279 mAh/g) for lithium-ion batteries’ cathode. Nevertheless, low conductivity, slack reaction kinetics, and large volume change during Li+ ions intercalation and deintercalation seriously limit the practical application of α-MoO3. Herein, we added a small number of CNTs (1.76%) to solve these problems in a one-step hydrothermal process for preparing the α-MoO3/CNTs composite. Because of the influence of CNTs, the α-MoO3 nanobelt in the α-MoO3/CNTs composite had a larger interlayer spacing, which provided more active sites and faster reaction kinetics for lithium storage. In addition, CNTs formed a three-dimensional conductive network between α-MoO3 nanobelts, enhanced the electrical conductivity of the composite, accelerated the electron conduction, shortened the ion transport path, and alleviated the structural fragmentation caused by the volume expansion during the α-MoO3 intercalation and deintercalation of Li+ ions. Therefore, the α-MoO3/CNTs composite cathode had a significantly higher rate performance and cycle life. After 150 cycles, the pure α-MoO3 cathode had almost no energy storage, but α-MoO3/CNTs composite cathode still retained 93 mAh/g specific capacity.


Introduction
Nowadays, to cope with the ever-increasing energy demands, it is imperative to develop energy storage technologies with low cost, high capacity, and long cycle life [1]. In typical energy storage technologies, rechargeable lithium-ion batteries (LIBs) stand out for their environmental friendliness, high energy density, high operating voltage, and no memory effect. However, with the rapid development of portable electronic devices, electric vehicles, grid applications, etc., researchers should continue to dive into the exploration of higher-capacity electrode materials to meet the increasing needs for higher energy-density storage, although LiCoO 2 , LiMn 2 O 4 , LiFePO 4 , and LiNi 0.33 Co 0.33 Mn 0.33 O 2 cathode materials have been commercialized [2]. Transition metal oxides with high reversible capacity and excellent electrochemical performance based on pseudocapacitive energy storage mechanisms are good candidates for cathode materials [3]. As a cathode material of LIBs, MoO 3 has piqued broad research interests due to its abundant resources, stable chemical properties, and high theoretical specific capacity [4][5][6][7]. Taking the widely studied thermodynamically stable orthorhombic stable a-phase MoO 3 (α-MoO 3 ) with anisotropic [5] as an example, its unique layered structure consists of two edge-sharing layers of [MoO 6 ] octahedra sharing corners along the crystal orientation [001] and [100], and a two-dimensional structure stacked by van der Waals forces along [010]. This two-dimensional structure of

Structure and Morphology Analysis
The α-MoO 3 /CNTs composite was prepared by a simple one-step hydrothermal method. Figure 1a shows the effect of CNTs on α-MoO 3 in the α-MoO 3 /CNTs composites, including the expanded interlayer distance α-MoO 3 structure and CNTs' three-dimensional conductive network in the α-MoO 3 /CNTs composite. This point is confirmed by the X-ray diffraction (XRD) results of α-MoO 3 /CNTs composite and pure α-MoO 3 . As shown in Figure 1b, the X-ray excitation source was monochrome Al Ka (hv = 1486.6 eV), power 150 W, X-ray beam spot 500 µm, and an energy analyzer through an energy of 30 eV. Due to the low content of CNTs, the α-MoO 3 /CNTs composite corresponds well to orthorhombic α-MoO 3 (JCPDS No. 05-0508, space group: Pbnm (62)) [24][25][26]. The α-MoO 3 /CNTs composite had better crystallinity without other impurity phases, and the (0k0) peak was very sharp, which further illustrates that the layered structure of α-MoO 3 held together along the b-axis and preferred orientation growth along the [001] direction. This is consistent with the long nanobelt shape in the SEM image. The SEM image ( Figure 1c) shows a thin and uniform nanobelt structure of the α-MoO 3 /CNTs composite with a length of about 1-6 µm [27][28][29][30]. Compared with pure α-MoO 3 , the composite nanobelts were shorter, and the TEM image in Figure 1d indicates the presence of bent CNTs between the α-MoO 3 nanobelts. The CNTs were uniformly distributed between the α-MoO 3 nanobelts to form a serviceable conductive network, which greatly increased the conductivity during the energy storage process. It is worth noting that the XRD patterns were partially enlarged, as shown in Figure 1e. Compared with the (0k0) peak position of pure α-MoO 3 , the peak position of the CNTs added α-MoO 3 /CNTs composite was significantly shifted to a smaller angle. This verifies that the CNTs embedded α-MoO 3 in the α-MoO 3 /CNTs composite had a larger interlayer spacing than the pure α-MoO 3 . Nanomaterials 2023, 13, x FOR PEER REVIEW 3 of 12 very sharp, which further illustrates that the layered structure of α-MoO3 held together along the b-axis and preferred orientation growth along the [001] direction. This is consistent with the long nanobelt shape in the SEM image. The SEM image ( Figure 1c) shows a thin and uniform nanobelt structure of the α-MoO3/CNTs composite with a length of about 1-6 μm [27][28][29][30]. Compared with pure α-MoO3, the composite nanobelts were shorter, and the TEM image in Figure 1d indicates the presence of bent CNTs between the α-MoO3 nanobelts. The CNTs were uniformly distributed between the α-MoO3 nanobelts to form a serviceable conductive network, which greatly increased the conductivity during the energy storage process. It is worth noting that the XRD patterns were partially enlarged, as shown in Figure 1e. Compared with the (0k0) peak position of pure α-MoO3, the peak position of the CNTs added α-MoO3/CNTs composite was significantly shifted to a smaller angle. This verifies that the CNTs embedded α-MoO3 in the α-MoO3/CNTs composite had a larger interlayer spacing than the pure α-MoO3.  Figure 2a is the Raman spectra comparison of the α-MoO3/CNTs composite and pure α-MoO3. Both the D-peak and G-peak are the Raman characteristic peaks of C-atom crystals, which were observed around 1300 cm −1 and 1600 cm −1 , respectively. The D and G bands in the Raman spectra can verify the presence of a small number of CNTs in the α-MoO3/CNTs composite. The characteristic bands of α-MoO3 in the composite were observed at 1006 cm −1 (Ag, νas Mo=O stretch) and 828 cm −1 (Ag, νas Mo=O stretch), which corresponds to the axially symmetric stretching vibration of the terminal Mo=O along the aaxis and b-axis, and the characteristic bands of α-MoO3 were observed at 676 cm −1 (B2g, B3g, νas Mo-O-Mo stretch) and 482 cm −1 (Ag, νas Mo-O-Mo stretch and bend), which corresponded to the bridge oxygen bond with weakly bound oxygen along the c-axis. The peak on the right side of Figure 2a is the D-band and G-band of the CNTs in the α-MoO3/CNTs composite, and the D-band and G-band frequencies were 1373 and 1620 cm −1 , respectively.  Figure 2a is the Raman spectra comparison of the α-MoO 3 /CNTs composite and pure α-MoO 3 . Both the D-peak and G-peak are the Raman characteristic peaks of C-atom crystals, which were observed around 1300 cm −1 and 1600 cm −1 , respectively. The D and G bands in the Raman spectra can verify the presence of a small number of CNTs in the α-MoO 3 /CNTs composite. The characteristic bands of α-MoO 3 in the composite were observed at 1006 cm −1 (A g , ν as Mo=O stretch) and 828 cm −1 (A g , ν as Mo=O stretch), which corresponds to the axially symmetric stretching vibration of the terminal Mo=O along the a-axis and b-axis, and the characteristic bands of α-MoO 3 were observed at 676 cm −1 (B 2g , B 3g , ν as Mo-O-Mo stretch) and 482 cm −1 (A g , ν as Mo-O-Mo stretch and bend), which corresponded to the bridge oxygen bond with weakly bound oxygen along the c-axis. The peak on the right side of Figure 2a is the D-band and G-band of the CNTs in the α-MoO 3 /CNTs composite, and the D-band and G-band frequencies were 1373 and 1620 cm −1 , respectively. The I D /I G = 0.876, indicating that the disorder of CNTs was high, which is consistent with the TEM image of Figure 1d. To determine the content of CNTs in the α-MoO 3 /CNTs composite, an thermogravimetric analysis of the composites was carried out. Figure 1b shows the TG and DTG curves of the α-MoO 3 /CNTs composite at a 10 • C/min heating rate in an air atmosphere. The apparent weight loss from 150 • C to 450 • C was the reaction of bound water in the composites, and the apparent weight loss from 450 • C to 550 • C could be ascribed to the decomposition of CNTs. Therefore, the content of CNTs was 1.76%, and a small amount of CNTs greatly influenced the structure of α-MoO 3 in the α-MoO 3 /CNTs composite. The XPS spectra of the α-MoO 3 /CNTs composite and pure α-MoO 3 are shown in Figure 2c  The ID/IG = 0.876, indicating that the disorder of CNTs was high, which is consistent with the TEM image of Figure 1d. To determine the content of CNTs in the α-MoO3/CNTs composite, an thermogravimetric analysis of the composites was carried out. Figure 1b shows the TG and DTG curves of the α-MoO3/CNTs composite at a 10 °C/min heating rate in an air atmosphere. The apparent weight loss from 150 °C to 450 °C was the reaction of bound water in the composites, and the apparent weight loss from 450 °C to 550 °C could be ascribed to the decomposition of CNTs. Therefore, the content of CNTs was 1.76%, and a small amount of CNTs greatly influenced the structure of α-MoO3 in the α-MoO3/CNTs composite. The XPS spectra of the α-MoO3/CNTs composite and pure α-MoO3 are shown in Figure   Due to the addition of CNTs in the hydrothermal reaction, some oxygen defects were introduced during the synthesis of α-MoO 3 . The α-MoO 3 had a larger interlayer spacing but still maintained the nanobelt structure in the α-MoO 3 /CNTs composite. If the content of CNTs is increased, the [MoO 6 ] octahedra structure will be more distorted and the nanobelts will be broken during the synthesis process. The α-MoO 3 /CNTs composite synthesized by adding a small number of CNTs in a one-step hydrothermal process is quite different from the microstructure of pure α-MoO 3 and will have a greater effective effect on the energy storage process.

Electrochemical Analysis
To further study the effect of CNTs on α-MoO 3 as a cathode material for lithium-ion batteries, the first three galvanostatic charge/discharge measurements of the α-MoO 3 /CNTs composite cathode and the pure α-MoO 3 cathode were performed at a current density of 50 mA/g between the potential of 1 and 3.25 V. The results were contrasted with cases from the other work, as shown in Table 1. From the galvanostatic charge/discharge curves of the α-MoO 3 /CNTs composite cathode in Figure 3a, it can be seen that there was a discharge voltage plateau at about 2.75 V in the first cycle. The plateaus corresponded to Li + intercalated in α-MoO 3 to form a Li x MoO 3 solid solution, and these discharge plateaus disappeared in the subsequent cycles. The Li + were reversibly intercalated and deintercalated in the α-MoO 3 /CNTs composite cathode, corresponding to the discharge voltage plateau at 2.25 V and the charging voltage plateau at 2.60 V, respectively. The initial dischargespecific capacity was 296 mAh/g, and the initial coulombic efficiency (ICE) reached 85%. The energy storage was primarily carried out by inserting and extracting Li + by α-MoO 3 in a composite cathode. In addition, the capacity of the pure α-MoO 3 cathode without CNTs constantly decayed from Figure 3c, indicating that the α-MoO 3 /CNTs composite had better electrochemical performance. At the same current density, the first discharge-specific capacity of the pure α-MoO 3 cathode was 268 mAh/g, but the first-coulomb efficiency was only 68%, which is probably the reason that the structure of pure α-MoO 3 was irreversibly destroyed during the intercalation of Li + [11,31]. In contrast, the structural failure of the α-MoO 3 /CNTs composite was caused by the volume expansion of α-MoO 3 during Li + storage [20]. Because of the embedding of CNTs, the α-MoO 3 nanobelt in the α-MoO 3 /CNTs composite had a larger interlayer spacing, which provided more active sites of Li + intercalation and deintercalation. Comparing the irreversible Li + storage part of the α-MoO 3 /CNTs composite cathode with that of pure α-MoO 3 by the galvanostatic charge/discharge curves, the lithium storage process of α-MoO 3 by the Li + intercalation and deintercalation of the [MoO 6 ] octahedral inter-layers and intra-layers positions, as well as the irreversible Li + storage, can be attributed to the Li + intercalated in the α-MoO 3 intra-layer positions, which could not be completely reversibly extracted [13,24]. The α-MoO 3 /CNTs composite cathode corresponded to only irreversible 0.23 Li + /Mo, while pure α-MoO 3 corresponded to irreversible 0.45 Li + /Mo. The pure α-MoO 3 corresponded to more irreversible Li + ion intercalation, which further verifies the improvement of CNTs on the reversible Li + ions storage part of α-MoO 3 . Comparing the cyclic voltammetry curves of the first three cycles at a scan rate of 0.2 mV/s of the α-MoO 3 /CNTs composite cathode with the pure α-MoO 3 cathode, Figure 3b shows that the reduction peaks of the α-MoO 3 /CNTs composite cathode appeared at 2.7 and 2.2 V. However, the reduction peak at 2.7 V disappeared in the subsequent cycle, which was caused by the irreversible intercalation of Li + into α-MoO 3 in α-MoO 3 /CNTs composite cathode and the generation of the interface phase between the composite cathode and the electrolyte. The results correspond to the galvanostatic charge-discharge curve. As Figure 3d shows, the reduction peaks of the pure α-MoO 3 cathode appeared at 2.6, 2.25, and 1.95 V, respectively, and then the peak position shifted significantly in the subsequent cycle, and the reduction peak appeared only at 2.1 V. These may have been due to the serious collapse of the layered structure of pure α-MoO 3 cathode after the first Li + insertion, and the reduction and instability of Li + intercalation active sites between and within layers of pure α-MoO 3 [32]. Compared with Nanomaterials 2023, 13, 2272 6 of 12 pure α-MoO 3 , the gap between the oxidation peak and the reduction peak was smaller, and the peak current was even larger in the CNTs added α-MoO 3 /CNTs composite cathode, indicating that the polarization phenomenon was smaller and the kinetic reaction was faster, respectively. the reduction peak appeared only at 2.1 V. These may have been due to the serious collapse of the layered structure of pure α-MoO3 cathode after the first Li + insertion, and the reduction and instability of Li + intercalation active sites between and within layers of pure α-MoO3 [32]. Compared with pure α-MoO3, the gap between the oxidation peak and the reduction peak was smaller, and the peak current was even larger in the CNTs added α-MoO3/CNTs composite cathode, indicating that the polarization phenomenon was smaller and the kinetic reaction was faster, respectively.   To explore the Li + storage mechanism of the CNTs added to the α-MoO 3 /CNTs composite cathode, we characterized the morphology and elemental chemical state of the Li + ion-intercalated α-MoO 3 /CNTs composite cathode after the first discharge. Figure 4a shows the TEM image of α-MoO 3 /CNTs first discharge to 1 V at a current density of 100 mA/g. It can be seen that the CNTs were uniformly distributed near the α-MoO 3 nanobelts. The intercalation of Li + during discharge increases the width of the α-MoO 3 nanobelts of composite, according to previous studies [35]. It is worth noting that the Li + intercalation composite cathode did not destroy the α-MoO 3 crystal structure, and the surface remained smooth during the discharge process. This further illustrates that the lithium storage mechanism is the topological redox reaction, which is different from the mechanism of the α-MoO 3 /CNTs composite as the anode for the lithium-ion battery. When the α-MoO 3 /CNTs composite was used as an anode electrode for lithium storage in a voltage window of 0.01-3 V, a clear Li 2 O crystalline layer was observed on the surface of the α-MoO 3 nanobelts, while no obvious Li 2 O crystalline layer was observed on the Li + ion-intercalated α-MoO 3 /CNTs composite cathode after discharge [10]. Amplifying the edge position of the nanobelts in the Li + -intercalated α-MoO 3 /CNTs composite cathode, as shown in the HRTEM image of Figure 4b Figure 4c indicates that the Mo and C elements from the α-MoO 3 /CNTs composite cathode, from the distribution of P element and EDS dark field image, can be further seen in the composite cathode Li + ion storage primarily through the α-MoO 3 /CNTs composite cathode of α-MoO 3 . According to previous studies, when the α-MoO 3 /CNTs composite is used as an anode electrode, composite-intercalated Li + converted into Mo and Li 2 O, and the Li + ions' deintercalation removed Li 2 MoO 3 amorphous based on the conversion reaction [10,35,36]. In the subsequent cycle process, the reversible reaction between Mo and Li 2 O with Li 2 MoO 3 occurs, which cannot return to the original α-MoO 3 /CNTs composite state, and more capacity loss occurs. Therefore, compared with the lithium storage mechanism of the topological redox reaction as the α-MoO 3 /CNTs composite cathode electrode of the lithium-ion battery, the first coulomb efficiency is lower when used as the anode electrode [9]. Figure 4d-f show the XPS images of the α-MoO 3 /CNTs composite cathode after the first discharge. Figure 4d XPS survey spectrum signifies the Mo, O, and C elements in the α-MoO 3 /CNTs composite, as well as Li, P, and F elements, from the intercalation of Li + into the composite cathode during discharge and the generation of the interface phase between the composite cathode and the electrolyte. Compared with the initial α-MoO 3 /CNTs composite cathode, in the Mo 3 d high-resolution XPS spectrum (Figure 4f), after Li + ions intercalation, the Mo 3d 5/2 and Mo 3d 3/2 of the discharged composites shifted to a small angle, which is attributed to the reaction: and to the formation of Li x MoO 3 [37][38][39]. Not only that, but the Li 1 s high-resolution XPS spectrum (Figure 4g), located in 54.9 eV and 56.7 eV, can be attributed to the formation of α-MoO 3 /CNTs composite cathode and electrolyte interface phases (CEI) and Li + ions intercalation into the α-MoO 3 layer of the formation. The current study conducted cyclic voltammetry tests at different scan rates to explain the lithium storage mechanism of the CNTs added α-MoO 3 /CNTs composite cathode. As shown in Figure 4g-i, the CV curve presented that the redox peak position did not shift significantly, and the curve shape was almost unchanged as the scan rate increased, indicating a better rate performance. When the scan rate was greater than 0.6 mV/s, the charge storage was dominated by the capacitance and less limited by the diffusioncontrolled redox reaction, indicating a greater Li + transfer kinetic. And when the scan rate was 1.0 mV/s, the diffusion control was less than 32.7%. The α-MoO 3 /CNTs composite cathode will lead to faster reaction kinetics and an improved rate performance.
The rate capability and cycle performance of the α-MoO 3 /CNTs composite cathode are shown in Figure 5a,b. When the current density was 100 mA/g, the composite cathode had an initial specific capacity of 266 mAh/g. The α-MoO 3 /CNTs composite cathode with 168, 139, 122, 112, and 77 mAh/g of specific capacity at the current density increased to 200, 300, 400, 500, and 1000 mA/g, respectively, and when the current density returned to 100 mA/g, the composite cathode specific capacity returned to about 180 mAh/g. This demonstrates that the composite structure was hardly destroyed as the current density increased. Compared with the pure α-MoO 3 cathode, when the current density was up to 1000 mA/g, the pure α-MoO 3 cathode had almost no capacity. Furthermore, when the current density returned to the initial 100 mA/g, the pure α-MoO 3 cathode specific capacity was much smaller than the initial specific capacity and was only 100 mAh/g. As a cathode electrode, CNTs hardly store Li + , but they effectively improve the Li + reversible storage characteristics of α-MoO 3 in the α-MoO 3 /CNTs composite and greatly improve the rate performance. Figure 5b shows the cycle performance of the α-MoO 3 /CNTs composite cathode and the pure α-MoO 3 cathode at a current density of 100 mA/g. After 150 cycles, the α-MoO 3 /CNTs composite cathode still had a specific capacity of 93 mAh/g, while the pure α-MoO 3 cathode had a specific capacity of only 34 mAh/g. The rate and cycle performance tests show that the capacity of the α-MoO 3 cathode will decay rapidly during the initial cycle process, while it will be relatively stable during the subsequent cycles. This is because the intralayer sites of Li + intercalation in α-MoO 3 cannot be completely reversible during the initial charge-discharge process. With the gradual occupation of the intralayer sites, the irreversible storage of Li + is improved [40]. The improvement of lithium storage electrochemical performance of α-MoO 3 by CNTs in the composite cathode was proved; the CNTs embedded into nanobelts to enlarge the interlayer spacing of α-MoO 3 , and the three-dimensional conductive network of CNTs improved the poor conductivity and the structural failure caused by volume expansion of pure α-MoO 3 during Li + deintercalation.  The current study conducted cyclic voltammetry tests at different scan rates to explain the lithium storage mechanism of the CNTs added α-MoO3/CNTs composite cathode. As shown in Figure 4g-i, the CV curve presented that the redox peak position did not shift significantly, and the curve shape was almost unchanged as the scan rate increased, indicating a better rate performance. When the scan rate was greater than 0.6 mV/s, the charge storage was dominated by the capacitance and less limited by the diffu- during the initial charge-discharge process. With the gradual occupation of the intralayer sites, the irreversible storage of Li + is improved [40]. The improvement of lithium storage electrochemical performance of α-MoO3 by CNTs in the composite cathode was proved; the CNTs embedded into nanobelts to enlarge the interlayer spacing of α-MoO3, and the three-dimensional conductive network of CNTs improved the poor conductivity and the structural failure caused by volume expansion of pure α-MoO3 during Li + deintercalation.  To further explore the reason why CNTs improve the electrochemical performance of the α-MoO 3 /CNTs composite cathode, the Li + diffusion kinetics of the composite cathode and pure α-MoO 3 cathode during charging and discharging were investigated by the galvanostatic intermittent titration technique. As shown in Figure 5c,d, the diffusion coefficient of α-MoO 3 /CNTs composite cathode was always higher than that of the pure α-MoO 3 cathode, which proves that Li + diffused faster in the α-MoO 3 /CNTs composite cathode. This may have been due to the presence of the CNTs' three-dimensional conductive network shortening the ion transport path. Meanwhile, we tested the electrochemical impedance spectroscopy for both the α-MoO 3 /CNTs composite cathode and pure α-MoO 3 cathode in Figure 5e. It shows that compared to pure α-MoO 3 , the semicircle diameter of the α-MoO 3 /CNTs cathode in the high-frequency region was smaller, corresponding to the charge transfer resistance at the interface between the electrode and the electrolyte. The fitting circuit is shown in the upper left of Figure 5e (Rct = 53.33 Ω), and the enhanced conductivity indicates that the α-MoO 3 /CNTs composite cathode had a higher electron conduction efficiency and faster kinetic reaction. In Figure 5f, compared with pure α-MoO 3 , the α-MoO 3 /CNTs composite cathode had a smaller diffusion resistance (Warburg factor, Z w ) in the low-frequency range, that is, the slope of the fitting curves of Z and ω −1/2 was smaller, which further verifies that the Li + diffusion in the composite cathode was faster. The Li + ion diffusion coefficient can be calculated according to the following formula: In Equation (2), D Li is the diffusion coefficient of Li + (cm 2 /s), R is the gas constant (8.314 J/Kmol), T is the temperature during the experiment (298 K), A is the electrode material area (cm −2 ), n is the number of electrons per mole of the active material transferred in the electrode reaction, F is the Faraday constant (96,500 C/mol), C is the lithium ion concentration (mol/L), and σ is the slope of Z ~ω −1/2 . The D Li = 9.3 × 10 −19 cm 2 /s in the α-MoO 3 /CNTs composite cathode. The CNTs were embedded inside α-MoO 3 nanobelts and cross-linked between α-MoO 3 nanobelts, which improved the conductivity of the composite cathode and accelerated the transmission of Li + ; it also effectively inhibited the volume expansion of α-MoO 3 during charging and discharging. These tests show that the α-MoO 3 /CNTs composite cathode had faster electron and ion transfer rates and faster reaction kinetics.

Conclusions
In summary, the CNTs were added by the one-step hydrothermal reaction to synthesize the α-MoO 3 /CNTs composite and thus effectively solve the poor conductivity and cycle stability of pure α-MoO 3 . The part of CNTs was embedded inside the α-MoO 3 nanobelts, which made the α-MoO 3 have a larger interlayer spacing and provided more active sites and faster kinetic reaction for lithium storage. Another part of CNTs formed a threedimensional conductive network between α-MoO 3 nanobelts, enhancing the electrical conductivity of the composite, accelerating the electron conduction in the energy storage process, shortening the ion transport path, and alleviating the structural fragmentation caused by the volume expansion during the α-MoO 3 deintercalation of Li + , leading to a significantly improved rate performance and cycle life of lithium storage.